Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask which is placed between the light source and the plate. With the advance of laser and computer technologies, laser sources have been increasingly used to directly expose a printing plate which is sensitized to a corresponding laser wavelength; photomask is unnecessary in this case.
Currently, most commercial lithographic plates require a development process after the plates being exposed and before put on press. A liquid developer is used to dissolve and clean off the non-exposed areas (for negative plate) or the exposed areas (for positive plates). Such a development process is time and labor consuming and generates wet waste. It would be desirable that such a tedious development process can be eliminated.
Lithographic plates which do not require any intermediate development of the photosensitive layer are disclosed in U.S. Pat. No. 3,997,349 (Sanders), U.S. Pat. No. 5,395,734 (Vogel), and U.S. Pat. No. 5,665,522 (Vogel et al). The photosensitive layer of the plates, upon irradiation, becomes more or less hydrophilic in the exposed areas. Such plates are called no-process plates. While the simplicity of these plates are attractive, such plates often suffer from background toning after a certain press run length.
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed. Therefore, such plates are also called no-process plates commercially. However, for the purpose of clarity and simplicity, in this patent, such plates will only be referred as on-press developable plates. Among the on-press developable lithographic printing plates are U.S. Pat. No. 5,258,263 (Cheema, et al), U.S. Pat. No. 5,407,764 (Cheema, et al), U.S. Pat. No. 5,516,620 (Cheng, et al), U.S. Pat. No. 5,561,029 (Fitzgerald, et al), U.S. Pat. No. 5,616,449 (Cheng, et al), U.S. Pat. No. 5,677,110 (Chia, et al), and U.S. Pat. No. 5,811,220 (Cheng, et al).
In order for an on-press developable plate to be useful, the non-hardened areas should be able to be cleaned off completely on press with ink and/or fountain solution during the initial press operation. Acceptable printed sheets should be achieved after the first several prints. Therefore, the non-hardened areas of the photosensitive layer should be able to be softened and dispersed by ink and/or fountain solution within seconds; the softened and dispersed areas of the photosensitive layer will be removed by the press offset roller and printing papers.
One formulation approach to designing negative-working on-press developable lithographic plates is to use an oleophilic photosensitive layer which is softenable and dispersible by press ink so that the unexposed areas of the photosensitive layer can be quickly removed on a wet press equipped with ink and fountain solution during regular press operation. Because press ink is usually a highly viscous and paste-like material, the photosensitive layer (unexposed) has to be capable of being quickly softened and dispersed upon contact with ink or combination of ink and fountain solution and being completely removed on press (on-press developability). At the mean time, the photosensitive layer has to be dry (non-tacky) enough so that the plate can be conveniently handled. To have good on-press developability while maintaining non-tackiness is an extremely challenging task. It is very difficult to formulate a photosensitive layer which has good on-press developability, non-tackiness, and durability. For example, for a photosensitive layer consisting of oleophilic polymeric binders, crosslinkable monomers, photoinitiators, and certain additives, increase in monomer content will give better on-press developability but will result in tackier coating; likewise, reduction in monomer content will give less tackier or non-tacky coating but will result in poorer on-press developability. Various formulation approaches have been used to address the above challenge.
U.S. Pat. Nos. 5,258,263 and 5,407,764 (Cheema, et al) describe an on-press developable lithographic printing plate using a photosensitive hydrophilic water-insoluble layer between a hydrophilic substrate and an oleophilic photosensitive layer to improve on-press developability.
U.S. Pat. No. 5,516,620 (Cheng, et al) describes incorporation of a plurality of microencapsulated developers in on-press developable lithographic plates. The microencapsulated developers may be integrated into the photosensitive layer, or may form a separate layer deposited atop the photosensitive layer. The microencapsulated developers after rupture helps soften the photosensitive layer and allows improved on-press developability.
U.S. Pat. No. 5,616,449 (Cheng, et al) describes the use of rubber and surfactant to enhance the durability and resolution of on-press developable lithographic plates. The rubber is preferably incorporated into the photosensitive layer as discrete particulate rubber.
U.S. Pat. No. 5,677,108 (Chia, et al) describes coating atop an on-press developable photosensitive layer a polymeric protective overcoat containing a hydrophilic polymeric quencher. U.S. Pat. No. 5,677,110 (Chia, et al) describes coating atop an on-press developable photosensitive layer a polymeric protective overcoat containing a water or fountain solution soluble or dispersible crystalline compound. Application of the overcoat reduces the tackiness of the photosensitive layer, among other suggested advantages.
U.S. Pat. No. 5,795,698 (Fitzgerald, et al) describes an on-press developable lithographic plate with addition of an amphoteric hydrogen bond-forming developability stabilizer in the photosensitive layer.
U.S. Pat. No. 5,811,220 (Cheng, et al) describes an on-press developable lithographic plate with incorporation of a hygroscopic microgel in the photosensitive layer. The microgel comprises crosslinked polymeric particulate beads with the outer surfaces of said beads being hydrophilic. The incorporation of hygroscopic microgel reduces tackiness and improves the developability of the plate.
Despite the progresses made in the aforementioned patents, none of the plates can provide satisfactory performance in all aspects, in terms of non-tackiness, good on-press developability, good press durability, and ease of manufacturing. The plates with microencapsulated developers, dispersed rubber or hygroscopic hydrogel require complicated manufacturing process. Putting an overcoat on a tacky plate reduces but does not completely eliminate the tackiness. Therefore, there is a continuing need for an on-press developable lithographic printing plate which has good on-press developability, no tackiness, simple manufacturing process, and good durability.
I have found, surprisingly, that addition of dispersed non-hydrophilic solid particles into the oleophilic photosensitive layer of an on-press developable lithographic plate can improve on-press developability and also reduce tackiness, or reduce tackiness without sacrificing on-press developability. Good durability and good shelf-life stability are also obtained with such a plate.
Although addition of dispersed particles (such as a pigment dispersion) in a photosensitive layer of conventional lithographic plates is well known, addition of non-hydrophilic solid particles (including a pigment dispersion) into the oleophilic photosensitive layer of an on-press developable plate is unexpected because of the following reasons. (1) Addition of non-hydrophilic solid particles would be expected to reduce on-press developability of the plate because such particles are expected to cause some residues on the substrate in the developed areas (leading to background toning). Since achieving good on-press developability has been a major challenge for on-press developable plates, any approaches which may adversely affect the on-press developability of the plates are generally avoided by the researchers. (2) Prior art literature on on-press developable plates has been teaching incorporating hydrophilic particles to improve both developability and non-tackiness.